Key Independent voters present even more of a challenge for Ryan, according to Pew. Only 35 percent of those voters view him favorably, compared with 52 percent who see him in an unfavorable light.

The Pew poll is part of a distressing trend for the vice president. In a September CBS/New York Times poll, Biden's favorability dipped to just 34 percent. In most September polling, it has stayed in the low- to mid-40s.

On the debate, most voters expect Ryan to come out on top of Biden. Forty percent chose Ryan as the victor, compared with only 34 percent for Biden.